(a) Field of the Invention
The present invention relates to a method of producing a flexible printed-circuit board covered with a coverlay film. The coverlay film is prevented effectively from curling and is used so that it and the flexible printed-circuit board can be located together easily. The applied coverlay is excellent in heat resistance, flexibility and dimensional stability.
(b) Description of the Related Art
Coverlay films are for protecting circuits made on flexible printed-circuit boards. Coverlay films are prepared by coating one surface of a base film, such as a polyimide film, with an adhesive, such as an acrylic adhesive or an epoxy adhesive, and are applied to a flexible printed-circuit board by bonding the surface coated with the adhesive to a surface of the flexible printed-circuit, board bearing a circuit using heat and pressure.
For the purpose of producing flexible printed-circuit boards of improved high efficiency, the use of conventional copper-clad laminates having a three-layer structure of copper foil/epoxy or acrylic adhesive/polyimide film has recently become replaced by the use of a two-layer structure of copper foil/polyimide film. However, using the above-described conventional coverlay films has ruined the advantages of copper-clad laminates of a two-layer structure, since, after covering with the coverlay, the properties of the flexible printed-circuit boards, including heat resistance, are dominated by the acrylic or epoxy adhesives used for bonding the coverlay films. In order to overcome such a problem, the use of polyimide adhesives was proposed as disclosed in Japanese Patent Application Kokai Koko (Laid-open) No. 3-205474.
However, the use of polyimide adhesives in preparation of coverlay films for flexible printed-circuit boards also encountered a problem that the coverlay films curled when dried after coating, thereby interfering with the proper location of them and flexible printed-circuit boards.
The causes for the problem inlcude the following (1) and (2).
(1) Polyimide adhesives contract severely when dried after application thereof on a base film, since polyimide for adhesives are generally poor in the solubility in solvents, as compared with acrylic or epoxy compounds for adhesives. Accordingly, a large amount of solvents is required for the preparation of polyimide adhesives, with the concentration of the polyimide adhesive components in the polyimide adhesives lowered.
(2) In production of two-layer flexible printed-circuit boards, for the purpose of preventing curling thereof, polyimides of low thermal expansion are generally used as base materials in order to make the circuit and the polyimide base similar in linear thermal expansion coefficient. While low thermal expansion is also required of the base films of coverlays, coverlay films themselves are apt to curl due to the difference in linear thermal expansion between the base film and adhesive layer thereof, since polyimides for adhesives do not have low thermal expansion.